Typically, active flow control involves injecting an energetic flow into a boundary layer to increase the momentum of the boundary layer flow thereby delaying separation of the boundary layer from the surface. In particular, active flow control on the intake lips of a jet engine has been proposed previously.
It has been suggested previously that active flow control could be provided by the use of perforations on the intake surface. A flow may be introduced through these perforations to re-energise the intake near wall boundary layer. These perforations may take the form of holes either perpendicular to the surface contour or angled in a uniform orientation. However, as shown in FIG. 1 such perforations produce plumes 10 which project into the mainstream flow 12 and generate a relatively weak horseshoe vortex 14 at the base, which is used to re-energise the boundary layer downstream. With this arrangement a high flow of air is required in order to re-energise the boundary layer sufficiently and this reduces the efficiency and performance of the engine. The source of this problem is that it takes a lot of power/energy to create the plumes, which in turn create relatively low powered horseshoe vortices that re-energise the boundary layer. Thus the previously-proposed mechanism generates high-powered jets and low powered vortices, but it is the vortices that do the useful work.
By contrast, U.S. Pat. No. 4,749,150 discloses a “Turbofan duct with noise suppression and boundary layer control” and describes the use of suction through the acoustic liner of an engine intake for boundary layer control. However, this invention also uses a lot of power to suck sufficient air from the boundary layer. Furthermore, it is also subject to blockage with debris such as dust and insects. For these reasons blowing, as opposed to suction, is preferred.
The present invention therefore seeks to address these issues.